Launched in 1941, Patek Philippe’s Reference 1518 became the world’s first perpetual calendar chronograph wristwatch produced in series.

Between 1941 and 1954, only 281 pieces of this Reference 1518 were made, the majority of which were in yellow gold.

What came after the Reference 1518 is the Reference 2499 and it is housed in a wider case with fluted lugs.

The dials of the Reference 1518 and Reference 2499 are similar because both are powered by the Calibre 13-ligne Q, highlights J. Michael Mehltretter in his book, Patek Philippe: Cult Object and Investment.

From what we understand, this is the Calibre 13-130, the ebauche by Valjoux and Victorin Piguet & Co. (Vallée de Joux).

However, the Reference 1518 had a 35mm diameter while the Reference 2499 is in a 37.7mm case. The Reference 2499 has a turned case middle and soldered-on band lugs.

The pink gold version of the Reference 1518 is obviously less common than the yellow gold model. According to auctioneer Phillips in Association with Bacs & Russo, approximately 44 pieces are known to be in pink gold. Even rarer are the stainless steel models of the Reference 1518.

“Stainless steel is by far the rarest case material that Patek Philippe chose to use during the mid-twentieth century for their complicated watches, considering the extremely high value of their movements.

“For many, steel complicated Patek Philippe watches are the ultimate in distinction to collect,” is what auction house Phillips puts it.

“After 40 years of international watch auctions and hundreds of books written on the subject, only four are known to exist. Of the four, three have at some point appeared at auction, with the last one [the last of the three] selling over a decade ago,” according to Phillips.

“The fourth example has never appeared at a public auction, and this is the watch to be offered. This watch, since its rediscovery in the 1990s, has always been exclusively held in private collections.”

This rightly explains why the fourth Reference 1518 will be appearing at the Phillips auction in Geneva in November 2016. It is also clear why Phillips declares this Reference 1518 as the ne plus ultra watch of the 20th century.

In fact, all three versions of the Patek Philippe Reference 1518 will be offered by Phillips.

More significantly, it comes as no big surprise why the Reference 1518 in stainless steel is valued in excess of CHF3 million, way above its “siblings” in pink gold (estimated value between CHF600,000 and CHF1.2 million) and yellow gold (estimated value between CHF250,000 and CHF500,000).